1. Field of the Invention
This invention relates to an arrester used for protecting a direct current power transmission thyristor valve from an overcurrent, and a direct current power transmission valve using such an arrester.
2. Description of the Related Art
Practical application of a direct current power transmission thyristor valve has been promoted in recent years with the increase in demand for electric power, and its operation voltage has increased from DC 125KV to 250KV and to 500KV to accomplish a high voltage and a large current.
To make an economical insulation design of such a direct current power transmission thyristor valve, an excellent arrester valve is necessary. However, such direct current power transmission thyristor valves are stacked into four stages and are electrically connected in series with one another. Therefore, the overall height becomes about 11 m to about 15 m. If the arrester used for protecting such a direct current power transmission thyristor valve from overvoltage is accommodated in a single insulator and is so stacked into four stages as to correspond to each stage in the same way as in the case of ordinary alternating current arresters formed by accommodating an arrester element in a single insulator, sufficient vibration resistance cannot be obtained. Therefore, as described in a papers of the '92 National Meeting of the Society of Electrical Engineering, S13-10, entitled "Arrester for AC/DC Conversion Station", a construction wherein the arrester is directly mounted to the main body of the thyristor valve has been proposed.
However, with the DC power transmission thyristor valve constructed in such a way as described above, in testing the withstand voltage of the thyristor valve, the thyristor valve arrester must be separated from the DC power transmission thyristor valve so as not to affect the insulation of the thyristor valve arrester. After this, they must be assembled together. These are rather troublesome procedures.
To solve the foregoing problems of the related art, the a thyristor valve arrester has been proposed as shown in the Japanese Published Patent Application No. 5-190256.
FIG. 8 illustrates the proposal, in which there are two stacked stages of arrester units 30a and 30b in view, each of which has an arrester element in a supporting bushing. The two stacked stages are mounted through a plurality of supporting bushings 31 on a base 32. There is also provided another assembly like the one shown in FIG. 8. These assembly stages are electrically connected in series to form a thyristor valve arrester of four arrester element stages. Each stage is connected with the respective stage of the thyristor valve through a conductor. However, the proposed thyristor valve arrester must have for each assembly set additional structures 33a and 33b for supporting the two stacked stages of arrester units 30a and 30b. The proposed thyristor valve arrester also must be isolated by a distance G of around 5 m from the thyristor valves 2A to 2D to be insulated As a result, the proposed thyristor valve arrester needs a large installation area and is complicated in the structure.
The thyristor valve arrester of the kind described above uses zinc oxide elements as arrester elements. Their insulation deterioration must be monitored by measurement of leakage current. The usual AC arrester can have a current transformer put in its grounding end, to easily detect a leakage current of around a few milliamperes. On the other hand, the thyristor valve arrester units 30a and 30b shown in FIG. 8, they are insulated from ground by the plurality of supporting bushings 31, and the leakage current cannot be easily measured. They also would be affected by a transient translocation surge current of around hundreds of milliamperes due to the translocation vibrating voltage of the thyristor valve. The ammeter therefore must be capable of measuring the leakage current of the thyristor valve arrester from as little as a few milliamperes to hundreds of milliamperes.